'cave in' definitions:

Definition of 'cave in'

(from WordNet)
noun
The sudden collapse of something into a hollow beneath it [syn: cave in, subsidence]
verb
Break down, literally or metaphorically; "The wall collapsed"; "The business collapsed"; "The dam broke"; "The roof collapsed"; "The wall gave in"; "The roof finally gave under the weight of the ice" [syn: collapse, fall in, cave in, give, give way, break, founder]

Synonyms of 'cave in'

From: Moby Thesaurus